Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D31: Focus Session: Quantum Magnets |
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Sponsoring Units: GMAG Chair: Ray Osborn, Argonne National Laboratory Room: 335 |
Monday, March 16, 2009 2:30PM - 2:42PM |
D31.00001: Spin Relaxation in Pure and Doped GGG Michael Schmidt, Thomas Rosenbaum, Daniel Silevitch, Gabriel Aeppli, Sayantani Ghosh, Y.K. Verma Geometric frustration in Gadolinium Gallium Garnet (GGG) leads to local regions of correlated spins that can be manipulated without affecting the background spin susceptibility. These ``quantum protectorates'' can be accessed via the non-linear response at milliKelvin temperatures using a hole burning technique. We study the effect of impurities on both the structure of the spin clusters and the dissipation spectrum in Neodymium-doped GGG crystals via pump-probe ac magnetic susceptibility and direct optical measurements. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D31.00002: Bose-Einstein Condensation of Triplons in Ba$_3$Cr$_2$O$_8$ A.A. Aczel, Y. Kohama, M. Jaime, L. Balicas, K. Ninios, H.B. Chan, H.A. Dabkowska, G.M. Luke By performing heat capacity, magnetocaloric effect, torque magnetometry and force magnetometry measurements up to 33 T, we have mapped out the T-H phase diagram of the S = 1/2 spin dimer compound Ba$_3$Cr$_2$O$_8$. We found evidence for field-induced magnetic order between Hc1 = 12.52(2) T and Hc2 $\sim$ 23.6 T, with the maximum transition temperature Tc $\sim$ 2.7 K at H $\sim$ 18 T. There are many qualitative features of the data suggesting that the transition at Hc1 corresponds to a Bose-Einstein condensation of triplons universality class. These include the apparent preservation of U(1) symmetry for applied fields below Hc1, a highly symmetric phase diagram, and an absence of any magnetization plateaus in the magnetic torque and force measurements. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D31.00003: Specific heat measurements in the novel frustrated quantum magnets SrHo$_{2}$O$_{4}$ and SrDy$_{2}$O$_{4}$ A. D. Bianchi, B. Prevost, N. Kurita, F. Ronning, R. Movshovich, T. W. Klimczuk, M. Kenzelmann, R. J. Cava We investigated the specific heat of the novel geometrically frustrated quantum magnets SrHo$_{2}$O$_{4}$ and SrDy$_{2}$O$_{4}$ to determine the nature of their ground states. We present a study of the magnetic field dependence of specific heat $C_p(T,H)$ measured in a dilution refrigerator between 0.1 K and 4 K and a PPMS between 2 and 50~K for magnetic fields $H$ between 0 and 9~T. We subtracted the phonon background $C_{\mathrm{ph}}$ by using a temperature dependent Debye temperature determined from measurements on the non-magnetic structural analogue SrLu$_2$O$_4$. After this subtraction, in SrHo$_2$O$_4$ we observed a broad anomaly in the magnetic specific heat $C_{\mathrm{mag}} = C_p - C_{\mathrm{ph}}$ centered at 0.5 K in zero field. At high fields, we found a broad peak centered at 0.35 K which decreases with rising magnetic field. SrDy$_2$O$_4$ in zero field has a broad anomaly at 1.2~K. The peak broadens with increasing $H$ and its amplitude decreases, and by 5~T it is completely suppressed. By 50~K, each ion in the Dy compound has recovered 21.5~J/mol~K of its spin entropy, which is comparable to the entire spin entropy of a free Dy ion of $R \cdot ln(2J+1)$, whereas we observe only 11.1~J/mol~K for SrHo$_2$O$_4$. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D31.00004: The Interplay of Quantum Criticality and Frustration in Columbite Ribhu Kaul, SungBin Lee, Leon Balents CoNb$_2$O$_6$ is a remarkable material. It can be modeled as a lattice of Ising chains coupled to each other in a frustrated anisotropic triangular lattice in the basal plane perpendicular to the chain direction. Applying a strong transverse field tunes the chains through a quantum phase transition into a paramagnetic phase. The interplay between two of the most interesting features of correlated quantum physics, quantum criticality and geometric frustration, produces a rich phase diagram which reflects the fundamental underlying quantum many-body physics. Using a variety of analytic and numerical techniques, we map out the phase diagram of this material in both transverse and longitudinal fields and provide a comparison with experiment. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D31.00005: Quantum phase transitions of the asymmetric three-leg spin tube Toru Sakai, Masahiro Sato, Kouichi Okunishi, Yuichi Otsuka, Kiyomi Okamoto, Chigak Itoi We investigate quantum phase transitions of the S=1/2 three-leg antiferromagnetic spin tube with asymmetric inter-chain (rung) exchange interactions. On the basis of the electron tube system, we propose a useful effective theory to give the global phase diagram of the asymmetric spin tube. In addition, using other effective theories we raise the reliability of the phase diagram. The density-matrix renormalization-group and the numerical diagonalization analyses show that the finite spin gap appears in a narrow region around the rung-symmetric line, in contrast to a recent paper by Nishimoto and Arikawa [1]. The numerical calculations indicate that this global phase diagram obtained by use of the effective theories is qualitatively correct. In the gapless phase on the phase diagram, the numerical data are fitted by a finite-size scaling in the $c=1$ conformal field theory. We argue that all the phase transitions between the gapful and gapless phases belong to the Berezinskii-Kosterlitz-Thouless universality class [2]. \\[0pt] [1] S. Nishimoto and M. Arikawa, Phys. Rev. B 78 (2008) 054421.\\[0pt] [2] T. Sakai, M. Sato, K. Okunishi, Y. Otsuka, K. Okamoto and C. Itoi, Phys. Rev. B 78 (2008) 184415. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D31.00006: Critical behavior study of antiferromagnetism in isostructural La$_{2}$CuO$_{4+\delta }$ and La$_{2}$NiO$_{4+\delta }$ Benjamin White, John Neumeier, A. Erb Neutron diffraction [1] and nuclear quadruple resonance [2] experiments coupled with theory calculations provide substantial evidence to support the widely-accepted belief that the two-dimensional Heisenberg model describes the antiferromagnetic interactions in La$_{2}$CuO$_{4}$ (S = $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} )$ and La$_{2}$NiO$_{4}$ (S = 1). The heat capacity critical exponent $\alpha $, which could provide further evidence, has never been studied in these two compounds because of the weak nature of the anomalies at T$_{N}$. [3] We will present heat capacity and high-resolution thermal expansion measurements of La$_{2}$CuO$_{4}$ and La$_{2}$NiO$_{4}$ single crystals, grown by the floating-zone method, and an analysis of $\alpha $ within the context of predicted values for a variety of universality classes. This material is based upon work supported by the NSF (DMR-0504769) and US DOE Office of Basic Energy Sciences (DE-FG-06ER46269) [1] Y. Endoh et al., PRB 37, 7443 (1988); G. Aeppli and D.J. Buttrey, PRL 61, 203 (1988). [2] P. Carretta, A. Rigamonti, and R. Sala, PRB 55, 3734 (1997). [3] T. Kyomen et al., PRB 60, 14841 (1999); K. Sun et al., PRB 43, 239 (1991). [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D31.00007: Physics of the spin gap in the $S=1/2$ Heisenberg antiferromagnet on kagome Invited Speaker: A combination of low spin and strong frustration makes the $S=1/2$ Heisenberg antiferromagnet on kagome a likely candidate for an unusual ground state and elementary excitations. Exact-diagonalization studies [1] on finite clusters point to a lack of magnetic order in the ground state and to an energy gap of order $J/20$ for $S=1$ excitations. The exact nature of the ground state and elementary excitations remains a subject of vigorous debate. Among the proposed ground states are chiral [2] and non-chiral [3] spin liquids and a valence-bond crystal (VBC) [4-5]; spin excitations range from deconfined spinons with a Bose [6] or Fermi statistics [2-3] to magnons [7]. We show that the system behaves as a collection of spinons, quasiparticles with $S=1/2$ and Fermi statistics, whose motion disturbs valence-bond order. Attraction between spinons, mediated by exchange, binds them into small, massive pairs of $S=0$ with a binding energy of $0.06 J$ [8]. The pair formation strongly suppresses the motion of individual spinons and makes the survival of the Singh-Huse VBC plausible. A spin excitation amounts to breaking up a pair into two (nearly) free spinons with $S=1$. The survival of the VBC is expected to lead to spinon confinement; however, small energy differences between various valence-bond configurations would make the confinement length large. \\[4pt] [1] Ch. Waldtmann et al., Eur. Phys. J. B \textbf{2,} 510 (1998).\\[0pt] [2] J. B. Marston and C. Zeng, J. Appl. Phys. \textbf{69,} 5962 (1991).\\[0pt] [3] M. B. Hastings, Phys. Rev. B \textbf{63,} 014413 (2000).\\[0pt] [4] P. Nikolic and T. Senthil, Phys. Rev. B \textbf{68,} 214415 (2003).\\[0pt] [5] R. R. P. Singh and D. A. Huse, Phys. Rev. B \textbf{76,} 180407 (2007).\\[0pt] [6] S. Sachdev, Phys. Rev. B \textbf{45,} 12377 (1992).\\[0pt] [7] R. R. P. Singh and D. A. Huse, arXiv:0801.2735. \\[0pt] [8] Z. Hao and O. Tchernyshyov, the subsequent talk. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D31.00008: Bound state of two spinons in a S=1/2 Heisenberg antiferromagnet on kagome Zhihao Hao, Oleg Tchernyshyov Elser et al. [1,2] identified a promising route to the ground state of the S=1/2 Heisenberg antiferromagnet on kagome via dimerized states, in which 3/4 triangles contain a valence bond. Quantum dynamics arises from the remaing ``defect'' triangles lacking a valence bond. We study an isolated defect on the Husimi cactus, a tree-like modification of kagome [1,3]. We show that the defect can be viewed as a bound state of two fermionic spinons with S=0. The bound state is small, on the order of 1.5 lattice spacings. It is localized and has a binding energy of 0.06 J relative to the 2-spinon continuum. No bound state is formed by 2 spinons with S=1. We argue that the pair-binding energy determines the spin gap of the kagome antiferromagnet. Our result for the gap agrees with the existing numerics [4,5]. [1] V. Elser and C. Zeng, Phys. Rev. B 48, 13647 (1993). [2] C. Zeng and V. Elser, Phys. Rev. B 51, 8318 (1995). [3] P. Chandra and B. Doucot, J. Phys. A: Math. Gen. 27, 1541 (1994). [4] Ch. Waldtmann et al., Eur. Phys. J. B 2, 510 (1998). [5] R. R. P. Singh and D. A. Huse, arXiv:0801.2735. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D31.00009: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 4:42PM - 4:54PM |
D31.00010: Order and Disorder in AKLT Antiferromagnets in Three Dimensions Siddharth Parameswaran, S.L. Sondhi, Daniel Arovas The models constructed by Affleck, Kennedy, Lieb, and Tasaki (PRL {\bf 59}, 799 (1987)) describe a family of quantum antiferromagnets on arbitrary lattices, where the local spin $S$ is an integer multiple $M$ of half the lattice coordination number. The equal time quantum correlations in their ground states may be computed as finite temperature correlations of a classical $\textsf{O}(3)$ model on the same lattice, where the temperature is given by $T=1/M$. In dimensions $d=1$ and $d=2$ this mapping implies that all AKLT states are quantum disordered. We consider the $d=3$ case where the nature of the AKLT states is now a question of detail depending upon the choice of lattice and spin; for sufficiently large $S$ some form of N{\'e}el order is almost inevitable. On the unfrustrated cubic lattice, we find that all AKLT states are ordered while for the unfrustrated diamond lattice the minimal $S=2$ state is disordered while all other states are ordered. On the frustrated pyrochlore lattice, we find (conservatively) that several states starting with the minimal $S=3$ state are disordered. These are a significant addition to the catalog of magnetic Hamiltonians in $d=3$ with ground states known to lack order on account of strong quantum fluctuations. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D31.00011: Global phase diagrams of frustrated quantum antiferromagnets in two dimensions: doubled Chern-Simons theory Cenke Xu, Subir Sachdev We present a general approach to understanding the quantum phases and phase transitions of quantum antiferromagnets in two spatial dimensions. We begin with the simplest spin liquid state, the Z$_2$ spin liquid, whose elementary excitations are spinons and visons, carrying Z$_2$ electric and magnetic charges respectively. Their dynamics are expressed in terms of a doubled U(1) Chern-Simons theory, which correctly captures the ``topological'' order of the Z$_2$ spin liquid state. We show that the same theory also yields a description of the variety of ordered phases obtained when one or more of the elementary excitations condense. Field theories for the transitions and multicritical points between these phases are obtained. We also survey experimental results on antiferromagnets on the anisotropic triangular lattice, and make connections between their phase diagrams and our results. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D31.00012: Stability of the U(1) spin liquid with spinon Fermi surface in 2+1 dimensions Sung-Sik Lee We study non-perturbative stability of a 2+1 dimensional critical spin liquid state, the U(1) spin liquid with a spinon Fermi surface. By mapping the spinon Fermi surface into an infinite set of 1+1 dimensional chiral fermions, we show that an instanton has an infinite scaling dimension for any nonzero N, where N is the number of spinon flavors. Therefore, the spin liquid state can be stable against confinement in physical systems, such as spin 1/2 magnets on the triangular lattice. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D31.00013: Finite-size scaling of string order parameters characterizing the Haldane phase Hiroshi Ueda, Hiroki Nakano, Koichi Kusakabe We have developed a numerical procedure to clarify the critical behavior near a quantum phase transition by analyzing a multi- point correlation function characterizing the ground state. The procedure focuses the gradient of the inversed-system-size dependence of the correlation function on a logarithmic plot. It requires only the correlation functions of several finite sizes under the same condition as a candidate for the long-range order. We apply the analysis to the string order parameter of the $S=1$ $XXZ$ chain with uniaxial single-ion anisotropy obtained by the density matrix renormalization group method. The present analysis gives precise estimates of transition points and critical exponents, $\nu$ and $\eta$, in Ising transitions, Gaussian transitions, and Berezinskii- Kosterlitz- Thouless transitions are consistent with results obtained from the analysis of the energy-level structure. This method will contributes much for a direct observation of quantum phase transitions. [Preview Abstract] |
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